This invention relates to a steering arrangement for an articulated vehicle, and more particularly to the physical arrangement and relationships of the pivot points for the steering cylinders which provide those steering characteristics required of an agricultural or farm tractor for operation in row crops. Basically, it is important that such a steering arrangement provide a good steering moment or torque because the tractor itself is heavy, which weight is often increased by the transfer onto the tractor's axles of a portion of the weight of the implement mounted on or towed behind the tractor, and because the draft load tends to resist articulation, and because the conditions of the terrain being traversed is frequently muddy or otherwise not conducive to steering maneuvers, particularly extreme steering movements such as those necessary at the headlands which sometimes require a full 180.degree. turn.
In addition to a good steering moment, a steering response which is uniform is highly desirable. It is apparent that use of an equal volume on each end of the individual steering cylinders, i.e. an equal area piston, would contribute to a uniform response. However, it is also apparent that the loss of available area on what would otherwise be the head end side of the piston would result in a reduction of the force contributing to the steering moment. Since the use of two cylinders, one on each side of the axis of articulation, will inherently require that one steering cylinder be extended while the other steering cylinder is contracted, the sum of the forces, i.e. pressure times the area on which it is acting, will be equal in either direction of turn. The problem is then not the force, but the variable moment arms upon which those two forces will act. The moment arm is a function of the geometry, specifically the location of the front and rear anchor points of the two cylinders with respect to the tractor articulation joint. Many factors affect this geometry, such as the space limitations and the availability of sound structural members capable of transmitting the steering torque between the front and rear frames.
The steering response is affected by the variable moment arm upon which each of the steering cylinders acts. In the turning mode, i.e. turning away from the straight ahead position, the response of the tractor improves whereas the available turning torque decreases. This is because the moment arm for the cylinder on the outside of the turn, which is expanding and thus has pressure acting on its larger head end area, is decreasing, while the cylinder on the inside of the turn, which is retracting and thus has pressure acting on its smaller rod end area, has a moment arm which is increasing. The result is a lower total turning moment towards the end of the turn. However, the response improves in this mode because for each degree of articulation the travel rate of the expanding cylinder is less than the travel rate of the retracting cylinder. Consequently, less total fluid is required for the same response toward the end of the turn as would be required at the straight ahead position.
In the returning mode, i.e. coming back from a fully articulated position to the straight ahead position, there is more available steering torque, but the response is slower. This is because the extended, but now retracting, cylinder has pressure acting on the smaller area side of its piston with a relatively short moment arm, while the retracted, but now expanding, cylinder has pressure acting on the larger area side of its piston with a relatively long moment arm. The resulting total steering torque is high but the steering response is slow because more total fluid is required to fill the cylinders for each degree of turn in restoring the tractor to its straight ahead position. With a hydrostatic steering system, in which the fluid going to the steering cylinders is metered through a hand pump connected to the steering wheel, this change in response from turning mode to returning mode is manifested in the operator being required to rotate the steering wheel more to return to straight ahead than was required to initially turn the tractor.
It is, therefore, an object of this invention to provide a steering arrangement for use with an articulated farm tractor having a hydrostatic steering system in which the difference in response between turning mode and returning mode is minimized, or at least substantially reduced.
It is also an object of this invention to provide a steering geometry for an articulated farm tractor which has good steering torque and a relatively uniform steering response.
It is a further object of this invention to provide such a steering geometry which is practical and which permits its use with a hydrostatic steering circuit having an operator controlled hand pump to meter fluid to the steering cylinders.